The present invention relates, generally, to a reprographic copying apparatus and, more particularly, to a control system for controlling the relative drive speeds of the media sheet transfer surface and a fusing roller assembly. In a transfer process such as conventional transfer xerography, a developed image pattern on a photoreceptor surface is transferred to a copy sheet which then passes through a fusing station so as to permanently fuse the toner image to the copy sheet surface, thereby preventing smearing or disturbance of the toner image by mechanical agitation or electrical fields. For this reason, and also for reasons of simplifying and shortening the paper path of the copier, and also for space savings, it is desirable to maintain the fusing station as close as possible to the transfer station. A particularly desirable fusing operation incorporates a roll-type fuser wherein the copy sheet is passed through a pressure nip between two rollers or a heated roller and a biased web. An example of this latter type of system is described in U.S. Pat. No. 4,689,471 assigned to the same assignee as the present invention.
However, when the fuser station is located near enough to the transfer station so that a lead portion of the copy sheet is in the fuser roll station simultaneously with the rear or trailing portion of that same copy sheet still being in contact with the photoreceptor, than a serious problem can arise. The problem is manifested in smears or skips in the unfused toner image which has been, or is being, transferred to the trailing portion of the copy sheet. This condition is caused by relative movement or slippage between the photoreceptor and the copy paper in those areas where they are still in contact, i.e., which have not yet been stripped away from the photoreceptor. A cause of such slippage is a speed mismatch between the nip speed of the fuser station (the speed at which the fuser is pulling the lead edge of the paper through the fuser) and the surface speed of the photoreceptor. If the fuser roll nip speed is slower, maximum buckle exists and the copy sheet can slip backwards relative to the photoreceptor. If the fuser roll is faster, the copy sheet can be pulled forward relative to the image on the photoreceptor. Eiether phenomenon can cause the aforementioned smears or skips in the toner image being transferred to the trailing area of the copy sheet.
An exactly equal velocity drive connection between the photoreceptor and the fuser is difficult, if not impossible, to maintain. Also, there is a further complication in that the actual sheet driving velocity at the fuser roll nip can change with changes in the effective diameter of the fuser driving roll. This can occur from replacement of the rollers, or changes in the applied nip pressure, materials aging, temperature effects, etc. Thus, equal speed is nearly impossible to maintain between the fuser roll nip and the photoreceptor surface in a commercial apparatus and may require increased maintenance and speed adjustment mechanisms.
Where the spacing between the fusing station and the transfer station is greater than the dimensions of the copy sheet, and a separate two-speed sheet transport is provided therebetween, then substantially different fuser roll nip speeds can be provided for as in U.S. Pat. No. 3,794,417, issued Feb. 26, 1974, to J. A. Machmer. However, this system has the noted disadvantages of requiring additional space, increased unfused image sheet handling, and also entails the additional complexity and expense of the additional transport mechanism.
It is known in the copying art to form a buckle in a copy sheet in its movement through the copier at various locations and for other functions. For example, it is known to interrupt the forward movement of a copy sheet with registration fingers and to form a buckle in the copy sheet by its continued feeding by upstream feed rollers to provide registration of the lead edge of the copy sheet before the copy sheet is fed into the image transfer station, e.g., U.S. Pat. No. 3,601,392, issued Aug. 24, 1971, to Merton R. Spear, Jr., et al. It is also known to provide or pre-form a buckle in a web of copy material to compensate for the braking of the web during a cutting operation in which the web is cut into individual sheets, e.g., U.S. Pat. No. 3,882,755, issued May 13, 1975, to Alan F. McCarroll. The later patent also illustrates that the copy web may be preformed into an initial convex buckle over an apertured surface and that air pressure may be utilized to expand the buckle when the web is stopped downstream thereof.
U.S. Pat. No. 3,774,907, issued Nov. 27, 1973, to Stephen Borostyan illustrates a vacuum sheet stripping device for removing copy sheets from the initial image support member and advancing them to a roll fuser, wherein the copy sheets assume a convex shape. A rotating cylindrical apertured vacuum is automatically cut off to the vacuum stripping member to release the copy sheet.
It is also known in the art to form a buckle specifically in the area between the transfer and fusing station. U.S. Pat. No. 4,017,065, issued Apr. 12, 1977 to Raymond E. Poehlein, discloses a speed mismatch compensation system which allows the fusing roll nip to be closely spaced from the transfer station of an electrostatographic copier by a distance less than the movement dimension of an individual copy sheet. The intermediate portion of the copy sheet is selectively supported and guided in a manner so as to form a buckle which accommodates a speed differential between the fuser roll nip velocity and the velocity of the photoreceptor. The fuser roll nip velocity is pre-set to always provide a somewhat faster speed than that of the photoreceptor. The speed mismatch is then accommodated by a reduction in the buckle rather than forward slippage on the photoreceptor. This allows a fixed and uncritical fuser roll drive which does not have to be adjusted relative to the photoreceptor surface drive.
In U.S. Pat. No. 4,025,187 issued May 24, 1977 to Taylor et al., a second type of buckle control system is disclosed that uses a reference time interval for feeding a sheet against a stop member to form a buckle in the sheet. After expiration of the time interval, the sheet separator is deactivated. A leading edge sensor and a buckle sensor feed signals to control the timing interval.
A drawback to the above described buckle control systems is that they are directed toward constantly maintaining the buckle at an exact configuration or are limited in the length of the sheet that can be accomodated. Precise control systems, typically using servo motors, are normally required. It would be desirable to provide a speed mismatch system which permits the buckle size to vary around an optimum size and which utilizes a simple drive system which can selectively drive one or the other of the two mismatched systems to periodically restore the buckle to an optimum configuration. Accordingly, the invention is directed to a system which utilizes a controlled buckle between two media drive systems. More particularly, the invention is directed towards a system for controlling the movement of a copy media, a portion of which is in frictional contact between a first and second rotational surface, and forms a buckle there between, the system including first drive means for controlling the rotational speed of said first surface, second drive means for controlling the rotational speed of said second surface, said first and second drive means resulting in a speed mismatch which creates a change in said buckle and a sensing means for sensing a predetermined buckle size in the portion of the copy media moving between the two rotational surfaces said sensing means adapted to generate an output signal for a predetermined length of time, said sensor output signal applied to one of said drive means so as to cause a change in the speed of said drive means whereby the said buckle configuration is restored to its original configuration.